This invention relates to liquid crystal devices and, more particularly, to such devices utilizing ferroelectric liquid crystal (LC) materials. Although the following description relates primarily to liquid crystal displays (LCDs), the primary intended application, the invention also contemplates use in other applications such as optical shutters.
A display device utilizing ferroelectric, chiral, smectic C materials has been suggested by N. A. Clark et al, Applied Physics Letters Vol. 36, p. 899 (1980). In this device, the smectic material is layered, and the layers are aligned perpendicular to the glass surfaces of the cell as shown in FIG. 1. The liquid crystal molecules lie flat on the surfaces and are restricted at the surfaces to only two positions (i.e., states S.sub.1 and S.sub.2) out of the cone of possible orientations (FIG. 2) that the chiral, smectic C state allows. The surfaces of the cell are closely spaced (about 1 .mu.m separation) so that the bulk of the sample follows the molecular orientation at the surface, thereby creating the two "surface-stabilized states" identified by Clark et al, supra. The influence of the surfaces also helps to suppress the helix of the chiral smectic C material so that the two states are not disrupted by pitch bands. The ferroelectric fixed dipole moment (M.sub.1,M.sub.2), which is inherent at the chiral center, points normal to the surfaces, up in one state (S.sub.1) and down in the other (S.sub.2). Hence, the device can be switched between the two states with a pulsed electric field applied via electrodes on the surface. The influence of the closely spaced surfaces causes the switched state to latch so that the state is maintained after the pulse is over. Thus, the device is bistable in that a latched state is maintained for time which is longer than the time between adjacent write and erase pulses or between adjacent refresh pulses, whichever is greater. Appropriately oriented polarizers are used to produce a visible contrast between the two states.
Fabrication of the above display may present difficulties owing to the required close (about 1 .mu.m) spacing of the cell surfaces. A ferroelectric display with a larger cell spacing in the range of 5 .mu.m to 10 .mu.m, as used in presently manufactured twisted nematic LCDs, would be desirable. However, when such larger spacings are used with conventional ferroelectric LC materials, a pitch band texture forms which renders the device less desirable. Even if the pitch bands are eliminated, the desired two-state behavior is not observed in thick cells (i.e., cells.gtoreq.2 .mu.m thick). Instead, at the end of the switching pulse, the material quickly reverts to a complex twisted state akin to the intermediate state sometimes seen in thin ferroelectric cells. In these twisted states, the molecular orientation varies around the smectic C cone as one goes from one cell surface to the other. What is needed in thick cells is a means for holding the molecular orientation in the bulk of the cell to those two orientations allowed close to the cell surfaces.